The magenta ink has probably dried out inside the printhead (not inside the cartridge) thereby causing a blockage.
Try using the deep cleaning cycle a few times which pushes the ink through with higher pressure than normal, so it will waste some ink but that's unavoidable.
If still blocked, you will have to remove the print head (if it's removable) and leave it to soak overnight in hand-hot, soapy water.
Epson printers are particularly prone to nozzle blockages if you don't print off a multi-colour document with them at least every two days so that a bit of every ink colour gets used to introduce fresh ink in to the print head, thereby pushing out the old ink before it dries to a stubborn paste. Especially important in hot...